The continued successful operation of a telephone network frequently requires on-site maintenance and diagnostic testing of a telephone line by field service technicians or craftspersons. Through experience and familiarity with long established telephone signalling characteristics, and with the aid of a variety of rudimentary test devices, the service technician has been able to electrically `splice` into a telephone link and `listen` to the line for telltale sounds of operational status. For example, one conventional procedure for detecting talk battery has involved the craftsperson bridging the tip and ring leads with a handset and listening for a sharp `click` indicating the presence of the 48 volt talk battery potential. Similarly, dial pulsing and voice signally have been detected by the craftsperson simply cutting in on the line and listening for activity.
As signalling formats and bandwidth utilization have expanded over recent years, however, this conventional aural-only monitoring approach to line testing and diagnostic evaluation may not only be unsuccessful, but may interfere with the transmission to be monitored.
More particularly, telephone lines have become a major communication link for a variety of communication signals other than voice; data communications between digital signal processing equipments commonly employ telephone lines as part of their communication network. In addition, with refinements in audio/digital signal processing systems, many telephone networks presently use an all digital format regardless of the type of information to be transmitted. As a consequence, a craftsperson who splices a handset into a telephone link for monitoring/testing purposes may be faced with the problem of not only being unable to hear or communicate (where the communication form is an all digital high data rate scheme), but the transient resulting from the butt-in can be expected to effect a burst-kill of the digital data traffic.